I think the reason is that for small quantities, like one tbsp of ingredient, the difference between the measures is not significant enough to make a difference. With larger quantities, like 2 Brit tbsp upwards, the difference of 3.5 ml *does* start to make a difference, and the two measures become more out of flunter unless you start to make the corrections suggested in the table.
(Well, I think I understand my own explanation, but if it is only as clear as mud [or chocolate!], do let me know, and I'll give it another go!)